Rat lung and liver microsomes mediated the biotransformation of the pulmonary toxin, 4-ipomeanol, to an alkylating metabolite. The enzyme-mediated microsomal alkylation required NADPH and oxygen and was strongly inhibited by carbon monoxide, which indicated the participation of a cytochrome P-450-dependent monooxygenase. Other studies with inhibitors including pyrazole, piperonyl butoxide, SKF-525A, and cobaltous chloride, and with the inducers phenobarbital and 3-methylcholanthrene, also were consistent with this view. The Km for the pulmonary microsomal alkylation pathway was more than 10-fold lower than for the hepatic microsomal pathway. There was no significant enzyme-mediated covalent binding of analogs of 4-ipomeanol lacking the furan moiety, suggesting that metabolic activation of the parent compound involves oxidation of the furan ring. Reduced glutathione prevented the microsomal alkylation by 4-ipomeanol, indicating the electrophilic nature of the alkylating metabolite.